It could be a jealous rival of Draco's. Maybe someone from another pure - blood family who has always been envious of the Malfoys' status and wealth. They see kidnapping Draco and Astoria as a way to bring shame to the family.
An old enemy of the Malfoy family from their Death Eater days could be the kidnapper. They might hold a grudge for something Draco's father did or for Draco's own actions during the wizarding war. Kidnapping Draco and Astoria would be a form of retaliation for past wrongs.
One possible plot could be that Draco and Astoria are kidnapped by a dark wizard who wants to use their pure - blood status for some dark ritual. Their friends, like Harry and Hermione, might then set out to rescue them. They would have to face various obstacles and dark magic along the way.
I also like 'The Malfoy Family Ties'. This fanfiction gives a detailed look into the daily lives of the Malfoy family. Draco is shown as a more complex character, trying to be a good father. Astoria is depicted as a loving mother, and Scorpius is just an adorable child caught in the middle of the family's growth and change. It's a great exploration of their family dynamics.
In some versions, Astoria is often depicted as more of a free spirit. She may introduce Draco to a new way of life that he finds exciting. Her charm and grace could also be factors. She might be the kind of person who doesn't put pressure on Draco like Hermione might have, in terms of his past actions or family history. This lack of judgment makes Draco feel more comfortable around her.
It could be Dr. Eggman. He's always after Sonic and would love to get his hands on him to stop Sonic from foiling his evil plans. He might want to study Sonic's speed and use it for his own machines.
It could be Zeref. Since Zeref has an interest in powerful magic users and Natsu is a strong dragon slayer, Zeref might kidnap Natsu to study his magic or use him in some dark plan related to his quest for immortality or power.
It could be an orc warband. Orcs have always been enemies of the elves, and they might see Legolas as a valuable captive either for ransom or to torture. Another possibility is a rogue wizard. Wizards who have turned to the dark side often seek powerful beings to experiment on, and Legolas, with his elven magic and skills, would be an interesting subject.
As mentioned before, Eggman is a likely candidate. He's always had his eyes on Tails' skills and would stop at nothing to use them for his evil plans. So he could be the one behind the kidnapping.
It could be a jealous rival. Maybe someone who has always been in competition with Shawn in some aspect of his life, like work or a hobby, and decides to take drastic measures by kidnapping him.
It could be a group of rogue Decepticons who are acting on their own. They might not follow Megatron's orders but still see value in kidnapping Arcee. Maybe they have a personal vendetta against her or think they can get rich by selling her to the highest bidder in the Cybertronian underworld.
It could be an envious rival. Maybe someone who's always been in the shadow of Osomatsu and his brothers and decides to take matters into their own hands by kidnapping Osomatsu. Another option is a deranged collector. This person might see Osomatsu as a rare item to add to their collection of oddities. Or it could be a vengeful spirit. In a more supernatural setting, a spirit with a grudge against the Matsuno family could kidnap Osomatsu as a form of revenge.